1. Field of the Invention
The present invention relates to industrial process control systems, and, more particularly, to a method of suppressing overshoot, i.e., passing through or exceeding a controller setpoint.
2. Background Art
The so-called usage of fuzzy logic has been described in the periodical "Plastic Technology"--June 1996 issue, wherein it is noted that "fuzzy logic tends to make controls think like you do".
Fuzzy logic has been utilized extensively in the area of industrial process control. It has been utilized in place of conventional controls because it is able to overcome some of the problems inherent in typical solutions. It is particularly concerned with the overshooting or undershooting of setpoints or process limits by improving response time. Process control installations have found extensive utilization of fuzzy logic inasmuch as it becomes useful in the areas of temperature or pressure control by dealing with events or characteristics that disturb the normal stability of the usual industrial control structure. When a change occurs during process, because of some extraneous source, it is necessary to take some form of corrective action.
It has been determined that if operational limits are set too loosely in order to control overshoot or undershoot characteristics, a system typically will he slow to react. On the other hand, if tighter standards are included, the system may respond more quickly and subsequently cause more considerable overshoot. Thus, fuzzy logic addresses these concerns by adapting to so-called "human language", such as "too hot", "too cold", "too slow", or "too fast". Simply speaking, fuzzy logic defines process limits with typical linguistic terms other than strict mathematical terms. Effectively processed values are compared to one another and with various degrees of importance assigned to each value with decisions made upon such comparisons. If speed, for example, is excessive, then it would be desirable, for example, to set the speed to "slow".
Single loop controller systems with an included overshoot suppression feature have been disclosed by several manufacturers. One of these is disclosed in a paper entitled "A Proportional Integral Derivative (PID) Controller with Overshoot Suppression Algorithm" by Yasuda, Mano, Mori, Azegami and Crotty from the Proceedings of the ISA90 International Conference and Exhibition--pp 1849-1857. This paper teaches a method to suppress process overshoot using a fuzzy logic control technique.
Overshoot suppression is embedded in a proportional integral derivative controller as a set of knowledge-based fuzzy rules which function to modify the controller internal setpoint so the controlled variable stays on a would-be or proposed response curve without oversshoot in the presence of process changes. This design requires prior setting of a parameter, "the effective process dead time", which is set by the controller's auto tuner. This design works for systems having a relative slow dynamic response. It is reported that if the loop has a very fast overshoot, suppression does not work and could even make the loop unstable.
A fuzzy temperature controller by Omron is disclosed as a fuzzy temperature controller in their Model E5AF. This one-quarter DIN controller combines fuzzy and proportional integral derivative control for fast response to process disturbances. In the disclosed system, advanced PID control with feed-forward circuitry provides optimal response during start-up and steady-state operation. The included fuzzy and PID control work together to correct a process upset quickly with minimal overshot. The arrangement allows fuzzy parameters to be changed to adjust the fuzzy control's impact on the process. Accuracy to .+-.0.3% of set value is claimed. Omron's design indicates that the system requires prior setting of three parameters: fuzzy intensity, fuzzy scale 1 and fuzzy scale 2 by the user. Should these be incorrectly set, or when the system dynamic response is too slow, the system may become unstable.
Accordingly, it is the object of the present invention to describe a method of predicting overshoot in an industrial process control system to enable the control system to take the necessary corrective act ion to reduce or eliminate such overshoot.